On Tue, Sep 08, 2015 at 02:17:13PM +0100, Chris Wilson wrote:
> In I915_READ64_2x32 we attempt to read a 64bit register using 2 32bit
> reads. Due to the nature of the registers we try to read in this manner,
> they may increment between the two instruction (e.g. a timestamp
> counter). To keep the result accurate, we repeat the read if we detect
> an overflow (i.e. the upper value varies). However, some harware is just
> plain flaky and may endless loop as the the upper 32bits are not stable.
> Just give up after a couple of tries and report whatever we read last.
>
> v2: Use the most recent values when erring out on an unstable register.
>
> Reported-by: russianneuroman...@ya.ru
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91906
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Michał Winiarski <michal.winiar...@intel.com>
> Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> Cc: Jani Nikula <jani.nik...@linux.intel.com>
> Cc: stable@vger.kernel.org
Still Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
> ---
> drivers/gpu/drm/i915/i915_drv.h | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 12870073d58f..51a88e70a6f7 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3402,13 +3402,13 @@ int intel_freq_opcode(struct drm_i915_private
> *dev_priv, int val);
> #define I915_READ64(reg) dev_priv->uncore.funcs.mmio_readq(dev_priv,
> (reg), true)
>
> #define I915_READ64_2x32(lower_reg, upper_reg) ({ \
> - u32 upper, lower, tmp; \
> - tmp = I915_READ(upper_reg); \
> + u32 upper, lower, old_upper, loop = 0; \
> + upper = I915_READ(upper_reg); \
> do { \
> - upper = tmp; \
> + old_upper = upper; \
> lower = I915_READ(lower_reg); \
> - tmp = I915_READ(upper_reg); \
> - } while (upper != tmp); \
> + upper = I915_READ(upper_reg); \
> + } while (upper != old_upper && loop++ < 2); \
> (u64)upper << 32 | lower; })
>
> #define POSTING_READ(reg) (void)I915_READ_NOTRACE(reg)
> --
> 2.5.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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